Author Topic: RF69HW and Antenna compatibility (receives but don't transmit)  (Read 2385 times)

fgomes

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RF69HW and Antenna compatibility (receives but don't transmit)
« on: December 02, 2016, 01:09:34 PM »
Hi, I have a system where Moteinos are used as sensors (with RFM69HW redios) and I'm using an ESP8266 as a Gateway. The ESP is connected directly to a RFM69HW module (is a prototype gateway, so the ESP and RFM are both assembled in a perfboard with some wires connecting them). This prototype should be assembled inside a small box, so there is not much space available, and the antenna should be inside that box. Initially I was using a coil antenna for the RFM69 and it worked, but the signal was much lower than using a single wire (with the nodes near, I only had about -40dB when usually with the wire antenna I have about -26dB). So I was trying different antenna configurations in order to have a better signal and still keep the antenna inside the box. I replaced the coil antenna by a 1/4 wave wire that must had some curves in order to fit inside the enclosure (at least one curve, so it got an L shape). When I connected the gateway it reported a very good signal received (at about -26dB, the same value of the straight wire), but then I notice that the sensor didn't receive the gateway acks. Investigatig a bit more I found that the gateway with that antenna doesn't transmit anything, but receives with a very good signal strenght.

Can it be some protection on the RFM69HW (antenna mismatch, stationary waves, ...)? Any idea where to search the cause? And any idea on how to be able to fit a small antenna inside the box without having this problem?

Best regards

Fernando

joelucid

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Re: RF69HW and Antenna compatibility (receives but don't transmit)
« Reply #1 on: December 02, 2016, 02:31:05 PM »
My guess is it's the CSMA limit in canSend. I would just recommend doing without that feature. Otherwise I'd recommend using a high quality antenna on the gateway so you can use smaller antennas on the sensors. At 915 MHz you can build a nice gw with a vertical dipole in an upright enclosure.

fgomes

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Re: RF69HW and Antenna compatibility (receives but don't transmit)
« Reply #2 on: December 02, 2016, 04:35:15 PM »
Hi Joe, thanks for your help!

I suspected also about the CSMA, but I've already commented the CSMA_LIMIT (the comparison with RSSI level made in canSend()) and the result was the same. It seems to be related with the antenna impedance or something similar, because it starts transmiting if I hold the antenna (wire) in my hand or if I straight the wire. Could it be related to the antenna wire to be close (a few mm away) at some points with other electronic boards (part of the perfboard, 1602 LCD module)? If they behave as a ground plane they can lower the antenna impedance, but I was expecting to observe less sensitivity and less transmitted signal, not shutting off the transmission completely. The helical antenna in the same position has a lousy signal but it works.

I'm using the RFM69HW with 20dBm output power, in the datasheet it says the maximum VSWR is 3:1, but didn't saw anywhere in the datasheet if it disables the transmitter or the PA if the VSWR is exceeded.

Best regards

Fernando


perky

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Re: RF69HW and Antenna compatibility (receives but don't transmit)
« Reply #3 on: December 02, 2016, 06:03:22 PM »
A wire a few mm above a ground plane would have some capacitance, for example a 1mm diameter wire 2mm above a ground plane for 2cm has about 0.5pF capacitance. My gut feel is it would mess up matching, but I've no experience of this.
https://www.emisoftware.com/calculator/wire-over-ground-plane-capacitance/
Mark.

fgomes

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Re: RF69HW and Antenna compatibility (receives but don't transmit)
« Reply #4 on: December 02, 2016, 07:12:18 PM »
Hi Perky, thanks!

I think that is is really an antenna impedance problem, due to the fact that putting a finger over the antenna (it is a cooper wire isolated, I put the finger on the plastic), the node starts transmitting. On the software side the node seems to be always transmitting, but the sniffer nearby (a moteino board) only sees the messages when I put a finger over the antenna.

Best regards

Fernando

joelucid

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Re: RF69HW and Antenna compatibility (receives but don't transmit)
« Reply #5 on: December 03, 2016, 01:58:11 AM »
Quote
I'm using the RFM69HW with 20dBm output power, in the datasheet it says the maximum VSWR is 3:1, but didn't saw anywhere in the datasheet if it disables the transmitter or the PA if the VSWR is exceeded.

There's no auto-disable feature I'm aware of. In fact I've used the transmitter without any antenna (the mother of all impedance mismatches) and it sends just fine. What you're describing is very puzzling since antennas are usually symmetric such that a good RX antenna is also similarly good for TX.

One thing I've seen, although only at higher bitrates, is that the signal is sometimes too strong to be received correctly. That happens since the REG_RSSITHRESH is set to 220 in rfm69.cpp, which essentially breaks the AGC mechanism if background noise is >-110 dBm. You could try say using 100 just for this test (translates to a minimum RSSI of -50 to trigger RX). Or reduce the powerLevel.

Not sure what to tell you otherwise. I have a very similar antenna in a sprinkler controller and it works fine. Maybe you could start with an upright wire and gradually transform it into your target shape and see when it breaks and how it behaves as you approach that boundary.

Joe


fgomes

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Re: RF69HW and Antenna compatibility (receives but don't transmit)
« Reply #6 on: December 05, 2016, 04:30:02 AM »
Hi Joe

Thanks again for your help! I soldered a short 50 ohm cable to the RFM69HW with an SMA connector at the other end in order to be easy to test with different antennas. I have manageded to make it work with a 1/4 wave antenna (simple wire) as far away as possible from the RFM69 module (I have it now glued to the bottom of the enclosure). If I put it a bit above that, it only transmitted if I put my finger over the antenna wire (that is isolated), but it received well with or without the finger on the antenna wire. I have replaced the RFM69 module, thinking that it could have some problem, but the result was the same. I thought if it could be some interference of the transmitted signal on the wires between the ESP8266 and the RFM69, but before assembling in a protoboard it was tested in a breadboard, with much longer wires. I tested it also without the antenna connected (just the cable with SMA connector) and it transmitted always, but of course with less signal.

So in summary, it is working, but the antenna placement can cause the node to stop transmitting, and the node receives well no mater where I place the antenna (with different signal levels but always receiving). My main worry is the stability, since I can't identify a reason for this problem. I have used many RFM69HW modules in the past and never had a similar problem - it is true that it is the first time I'm placing the antenna inside the enclosure, in every node I have, I used an external SMA antenna or the simple 1/4 wave straight wire going outside of the enclosure.

Best regards

Fernando

john4444

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Re: RF69HW and Antenna compatibility (receives but don't transmit)
« Reply #7 on: December 05, 2016, 01:20:45 PM »
Hi Fgomes,
The symptom "it works when I touch it" is often an RF gounding issue.
I would first look for a poor ground connection.
The poor connection does not have to be near the RFM69 antenna.
There could also be an issue at the power ground.
Next I would check for a poor/intermittent connection to the antenna.

Good Luck, John
John AE5HQ

Felix

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Re: RF69HW and Antenna compatibility (receives but don't transmit)
« Reply #8 on: December 05, 2016, 02:02:56 PM »
Thanks John - Yes it sounds very similar to what's discussed right now in this thread.